offsetof(3) is not documented so here's a man page for it. Is the text below enough or should it be more verbose?
.Dt OFFSETOF 3 .Os .Sh NAME .Nm offsetof .Nd offset of a structure member .Sh SYNOPSIS .Fd #include <stddef.h> .Ft size_t .Fn offsetof "type" "member" .Sh DESCRIPTION The .Fn offsetof macro expands to an integer constant expression and yields the offset, in bytes, of the field .Ar member from the start of the structure .Ar type . .Pp A compiler error will result if .Ar member is not aligned to a byte boundary (i.e. it is a bit-field). .Sh STANDARDS The .Fn offsetof macro conforms to .St -ansiC .
